It is basically because the mind has become habitual to the sound and sight since time immemorial, maybe from the beginning of creation perhaps. Because of imagination only the mind has gone out of control. This is the basic technique of the bhakti marga. Using the same imagination the mind is to be brought back to its concentrated position, the composed position.
So rituals and form worship or formlessness, they are all prescribed so that you can begin somewhere if you need an anchor, so that your mind can loose all other imaginations and become single-pointed into one imagination. Even it is advised during meditation also, if there are thousands of thoughts coming, you are unable to loose all the thoughts, try to stick to one particular thought before giving up all such thoughts. So that is how the worship of an idol in the temple and rituals are all prescribed.
You see God has never demanded whether he wants milk, gold or silver. What is needed is that your mind needs to be there. If you look into all scriptures, all sayings of all Saints, they have always advised, "Apply your mind and worship". Just like you go to a place of worship, you try to do some ritualistic worship. While doing so imagine if your mind is running and going around a shopping complex somewhere in the town; such a worship becomes useless, a waste of time and energy. So if you are offering some worship your mind also has to be there. It doesn't really matter whether you are going to pour a glass of water or a glass of milk on the idol. What is important is that your mind also has to be there.
But yet again why this has been given? Just one example: you cannot explain the space as it is. If a child asks, "What is space, does it really exist?" you cannot show it as it is. Still you would have to give some explanation to make it to begin. Just like when we went to the first standard of primary school, our teachers said in that school, "The place in between these two walls is known as space". Well as we grew up we went to the next classes, came to the university level, and as our knowledge and wisdom grew we understood that the same space exists everywhere. But now if I tried to claim that the space exists only in the classroom of my primary school in the town where I studied, everybody would laugh.
In the same way, to begin with you show an idol, a photograph, a form, give it an anchor to the mind, tell, "This is God"; you prostrate, you offer your worship. So it is expected that your concentration shall go there and gradually as you grow, if your worship is going to be real by applying all your mental concentration there, definitely you would gain that knowledge and wisdom that the same God exists everywhere and there is really no conflict. That is why my Guru always said, "You can believe God in any form or formlessness, but what is needed is that do not condemn others' beliefs, that is important". So that is the importance of all rituals, idol worships that has been taught as form and formlessness.